Commercial production of hydrogen peroxide in significant quantities has been performed using, mainly, one of three processes, viz:
i ) the formation of barium peroxide from the oxide heated in oxygen followed by acid dissolution yielding hydrogen peroxide; PA1 ii) the electrolytic anodic oxidation of sulphuric acid or its salts to peroxydisulphates which hydrolyze to yield hydrogen peroxide; and PA1 iii) the cyclical catalytic oxidation then reduction of organics, typically anthraquinone. PA1 i) a greater degree of complexity in the process due to differing electrolytes; PA1 ii) a requirement for acid resistant materials including anodes, gaskets, and other cell components; these materials can be much more expensive than materials suitable for an alkaline electrolyte; and, PA1 iii) a higher energy demand compared to alkaline anolyte cells by nature of acidic solutions in electrolytic cells. PA1 (a) introducing an aqueous anolyte between an anode and an anion permselective membrane surface of a bipolar membrane; PA1 (b) introducing an aqueous catholyte between a cation permselective membrane surface of a bipolar membrane and a first surface on a gas-diffusion cathode; PA1 (c) introducing oxygen-containing gas to a second surface on said gas-diffusion cathode; PA1 (d) connecting said acid resistant anode and said gas diffusion cathode with an external power supply for causing: PA1 (i) the oxygen to be reduced at said diffusion cathode to produce HO.sub.2 - ions within said aqueous catholyte, PA1 (ii) the hydroxyl OH- ions in said aqueous anolyte to be oxidized to produce oxygen and water within said aqueous anolyte, PA1 (iii) the water in said bipolar membrane to be dissociated into hydrogen ions H+ and hydroxyl OH- ions, PA1 (iv) the dissociation produced OH- ions, to move through the anion selective surface of said bipolar membrane to the aqueous anolyte whereupon said OH- ions maintain electroneutrality by replacing anodically oxidized OH- ions of said aqueous anolyte, and PA1 (v) the dissociation produced hydrogen ions (H+), to move through the cation selective surface of said bipolar membrane to the aqueous catholyte whereupon said hydrogen ions (H+) react with the cathodically produced HO.sub.2 - ions to produce hydrogen peroxide within said aqueous catholyte.
Although another process route for the production of hydrogen peroxide, viz, by the cathodic reduction of oxygen has been known for some time, such a process has only recently been utilized commercially for producing hydrogen peroxide solutions. The first of the commercial processes listed above is labour intensive and no longer of importance. The second process has high energy demand and, thus, led to the commercialisation of much lower energy demand anthraquinone processes. Anthraquinone processes usually produce solutions containing 70 to 90% hydrogen peroxide by weight which reduces transportation and storage costs; this is important since the complexity of these processes results in relatively high capital and maintenance costs which favour large production merchant plants. The theoretically low energy demand cathodic oxygen reduction process has received considerable attention recently, especially as the art of making gas-diffusion cathodes suitable for hydrogen peroxide production has progressed; this electrochemical route is potentially very simple to operate although the product is so far only a dilute caustic peroxide solution.
Dilute caustic peroxide solutions are particularly suitable for use in the wood pulp bleaching industry. In addition to the bleaching of woodpulp, alkaline solutions of hydrogen peroxide are suitable for other bleaching applications and chemical bleaching operations. Electrochemically produced hydrogen peroxide in low concentrations may be used without further concentration in such bleaching operations, and hence, on-site electrochemical hydrogen peroxide production has been contemplated for supplying hydrogen peroxide at wood pulp plants for bleaching.
Several approaches have been patented for the electrolytic production of hydrogen peroxide by cathodic reduction of oxygen. Yu-Ren Chin (SRI International, Report No. 68B, March 1992) summarizes the most important patents and presents economic comparisons between anthraquinone processes commercialised and cathodic oxygen reduction process. However, a major disadvantage of the presented methods of electrolytic preparation of alkaline peroxide solutions is that the inherent caustic to peroxide ratio (by mole) is larger than 2, which limits its end use applications.
In fact, in a typical electrochemical cell for the production of hydrogen peroxide in an alkaline electrolyte such as sodium hydroxide, the cathode reaction yields: EQU H.sub.2 O+O.sub.2 +2Na.sup.+ +2e.fwdarw.NaOH+NaHO.sub.2 ( 1)
Hence, it is evident that the minimum molar ratio of sodium ion to peroxide is 2.
Jasinski and Kuehn (U.S. Pat. No. 4,384,931; May 24, 1983) recognised the advantage, for lower caustic peroxide ratios, of an acidic anolyte. They describe an electrolytic cell having two electrolytes, one acidic, one alkaline, separated by a membrane permeable to positive ions (cation selective membrane). An acidic aqueous anolyte is introduced between an acid resistant anode and the cation selective membrane; an alkaline aqueous catholyte is introduced between a gas-diffusion cathode and the membrane; and an oxygen containing gas is introduced to the outside of the gas-diffusion cathode. Water in the acidic anolyte is electrolysed to form oxygen, hydrogen ions and electrons: EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e.sup.- ( 2)
Electrical neutrality requires that the hydrogen ions (H.sup.+) migrate toward the cathode through the cationic selective membrane and into the catholyte.
At the cathode, oxygen diffuses through the gas-diffusion cathode and reacts with the hydrogen ions (H.sup.+) migrating through the membrane from the anolyte and with sodium ions present in the catholyte to form sodium hydroxide and hydrogen peroxide by either of the reactions: EQU 2H.sub.2 O+2O.sub.2 +2H.sup.+ +2Na.sup.+ +4e.fwdarw.2NaOH+2H.sub.2 O.sub.2( 3) EQU 3/2O.sub.2 +3H.sup.+ +Na.sup.+ +4e.fwdarw.NaOH+H.sub.2 O.sub.2( 4)
However, the hydrogen ions (H.sup.+) migrating into the catholyte also neutralizes caustic as caused by the reaction; EQU NaOH+H.sup.+ .fwdarw.H.sub.2 O+Na.sup.+ ( 5)
Equations (3), (4) and (5) lower the caustic to peroxide ratio below 1.0 since the sodium ion of equation (5) again reacts to produce more peroxide. However, some of the disadvantages of the electrolytic cell with acidic anolyte may be listed as follows:
Bipolar membranes are composite membranes consisting of three parts, a cation selective region, an anion selective region and the interface between the two regions. When a direct current is passed across a bipolar membrane with the cation selective side toward the cathode, electrical conduction is achieved by the transport of H.sup.+ and OH.sup.- ions which are obtained from the dissociation of water which occurs at the interface under the influence of an electric field. Bipolar membranes are described, for example, in U.S. Pat. No. 2,829,095 to Oda et al, in U.S. Pat. No. 4,024,043 (single film bipolar membranes), and in U.S. Pat. No. 4,116,889 (cast bipolar membranes).
Paleologou and Berry (U.S. Pat. No. 5,006,211; Apr. 9, 1991) applied electrodialysis with bipolar membranes to the dealkalization of caustic peroxide solutions such as those produced by the reduction of oxygen in electrolytic cells (e.g. the Dow on-site peroxide generator, U.S. Pat. Nos. 4,224,129 and 4,317,704). Two compartment unit cells (alternating cation and bipolar membranes) and three compartment unit cells (alternating bipolar, anion, and cation membranes) are described for the dealkalization of typical 2:1 caustic/peroxide solutions with the co-production of a caustic solution suitable for recycle to the peroxide generator.
The disadvantage of the electrodialysis approach for dealkalization of a generated alkaline peroxide solution is the addition of another process system to an on-site electrolytic peroxide production plant; overall capital cost will be higher for the generating electrolysis system plus electrodialysis system, the overall energy demand will be higher in spite of the good efficiency of typical electrodialysis systems, and the increased number of equipment items will necessarily increase manpower requirements for operations and maintenance.
The most commonly referenced applications involving bipolar membranes are generally of the electrodialytic type, although direct electrolytic application is known. However, a bipolar membrane could not be used in every type of electrolytic cell such as those peroxide generators which rely on flow of alkaline anolyte through a porous diaphragm into the cathode chamber which is filled with composite carbon chips as a high surface area cathode; a bipolar membrane would not simply replace the porous diaphragm which is also of a special structure to ensure an even flow distribution to the cathode bed. Since such a peroxide generator is the only commercialised electrolytic cell, for cathodic reduction of oxygen to produce hydrogen peroxide, and since bipolar membranes are most generally associated with electrodialytic type applications, then the use of bipolar membranes in the electrolytic production of hydrogen peroxide by the cathodic reduction of oxygen is not readily apparent.